The present invention relates to a method for manufacturing a wound electrical capacitor with long service life comprising metallic layers conducting the electrical current and dielectric layers of thermoplastic plastic material which do not conduct the electrical current, disposed between said metallic layers. The invention also relates to a capacitor produced according to said method.
The manufacture of wound electrical capacitors comprising alternating metal layers and dielectric layers, for example, of polypropylene and optionally of other polymers, is already known from numerous publications. Reference may be made to European Pat. No. 0,001,525 as representative of all these publications. For reasons of space, recourse has been made to reducing the thickness of the metal layers substantially by depositing the metal in vacuo as extremely thin coatings directly on the dielectric layer of thermoplastic plastic material. As a result of this it was possible to reduce substantially the size of capacitors manufactured with such metallized dielectric films.
However, in the case of capacitors with the extremely thin metal layers produced by evaporation coating, it has been found that extremely undesirable capacitance losses are observed, in particular when the capacitors are used for alternating voltages over a prolonged period. These capacitance losses are the consequence of local destruction of the metallic layer caused by electrochemical corrosion, the metallic conductive aluminum forming the metal layer being converted into non-conductive polycrystalline aluminum oxide. As the metal area available decreases, the capacitance of the capacitor also decreases.
The phenomenon is observable as numerous, virtually circular metal-free sites inside the metallic layer whose frequency and growth rate depend, inter alia, on the magnitude of the operating voltage of the capacitor, on the frequency of the alternating current and on the operating temperature of the capacitor. It is known from an investigation by Taylor published in "IEEE Transactions on Electrical Insulation", Vol. EI-19, No. 4, Aug. 1984, that the corrosion of aluminum in metallized film capacitors is invariably connected with the presence of water or moisture between or in the winding layers of the capacitor under investigation.